As a general rule, do not use the serial/Oxford comma: so write ‘a, b and c’ not ‘a, b, and c’. But when a comma would assist in the meaning of the sentence or helps to resolve ambiguity, it can be used – especially where one of the items in the list is already joined by ‘and’:

The serial comma, dubbed the Oxford comma because of its use by writers and editors based at the University of Oxford, had been waning in popularity. For example, most journalists in Canada and the U.S. who follow the AP or CP stylebooks do not use it.

The Canadian Press Stylebook has this to say: "Put commas between elements of a series but not before the final and, or or nor unless that avoids confusion. But reactions among the grammar police on Twitter seem to be divided, with strong opinions on both sides. All this over a punctuation mark.

The serial comma has had other brushes with fame, of course, even inspiring a song by hipster band Vampire Weekend. No semicolon, em dash or tilde can say that.